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It’s always a good idea to include the mains VAC at the wall outlet, and the VAC across the pilot lamp terminals (ie heater voltage 6.3V).
I the...

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Electrolytic cap metal bodies aren’t necessarily at 0V common. With those used for the negative power supplies etc, touching the cap metal may result...

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How about if the input stage anode is scoped?
Has a different valve been tried?

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My take is that sort of thing is usually the tonestack / bright cap emphasising distortion of an imperfect sine from the sig gen.
if the treble and...

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I was thinking of the expected operating life figure in product data sheets.
It’s usually a worst case, at max rated voltage and temperature,...

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Mouser etc usually carry several lines of 4.7uF 500V film caps. Yes they’re more expensive than ecaps, but should a lot (eg last 10 times) longer.

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It seems reasonable to assume that the PT is receiving its intended mains voltage when the heater voltage (amp at normal idle loading) is 6.3VAC....

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On one hand, USB is a generic 5V standard, on the other some USB ports (ie providing a 5V charging source such as a wallwart or computer) can provide 2A,...

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Haha, I thought that (DC) anode mixing worked fine, I tried several methods on my Fender type amp build, and that seemed as good as any other. That’s...

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Maybe something about the footswitch cable has changed, leading to it having eg higher inductance?

There’s often low pass filtering on...

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Bruce Zinky designed a bunch of Fender amps in the 90s, the Custom Vibrolux for one included anode mixing of the channels, its schematic is dated Oct 1994....

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It was ‘anode or cathode’ current; cathode current will be a milliamp or 2 higher than at the anode. As it also includes screen grid current....

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Maybe a bad heater wiring connection between V3 and V2?
What anode or cathode currents are the output valves drawing at idle?
When measuring voltages...

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That’s a very good point

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Yikes, I’m dreading the day when my cancer finds its way around the hormone therapy I’m on

Zytiga (Abiraterone) was what I...

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If that means the reservoir cap, it probably wouldn’t be a ideal choice, as it may be the noisiest point of a typical amp’s 0V common arrangement....

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A good application for LED bias....

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Anechoic chamber in regard of external electromagnetic interference, rather than acoustics / mechanical vibrations.
In Cheshire, some testing companies...

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Yes, embarrassingly it happened on an early build of mine, then it crops up every so often on one forum or another, eg “my 5E3 clone is redplating,...

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Bias is the voltage differential between control grid and cathode.
With cathode biased stages, if the grid loses its reference to 0V common, the DC...

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It depends on the application really.
A type 3 (crossline) master can be fine for trimming down the amp’s on-stage sound level. But a basic one...